Treatment of viruses and bacteria to render them inactive is important in various areas, including sanitisation of waste water or sewage, and also in the field of parenteral medicines such as vaccines. For instance, many vaccines are based on microorganisms which are inactivated to ensure that live infectious material is absent from the final vaccine. Failure of inactivation can present severe safety risks, as known from “the Cutter incident” in the 1950s where inactivation of poliovirus failed.
Inactivation treatments are typically based on chemical means. Chemical treatments include the use of detergents, formaldehyde (usually as formalin), β-propiolactone (BPL), glutaraldehyde, ethyleneimines, phenol etc. These inactivators have been used for many years e.g. see reference 1.
Some of these inactivators are also useful for degrading nucleic acids. This degradation can play a role in inactivating the viruses or bacteria themselves, but it can also be useful for eliminating residual nucleic acids from any cell substrate which has been used during growth. For example, viruses can be grown in cell culture to provide material for preparing vaccines and during manufacture it is usual to include a step to degrade any residual nucleic acids from the cell culture substrate, thereby removing potentially oncogenic material. Reference 2 describes the use of BPL for both inactivating viruses and degrading host cell DNA.
The best inactivator varies according to the particular microorganism. For instance, differences in viral morphology (size, capsidation, envelope, etc.) lead to differences in inactivation sensitivity e.g. see Appendix 2 of reference 3. Although some treatments are universally able to inactivate microorganisms (e.g. very harsh heat, disinfecting agents, strong UV light or radiation), these also destroy immunogenicity and so are inappropriate for preparing effective vaccines. Inactivation treatments are instead chosen so that they are effective for the microorganism in question while retaining vaccine immunogenicity. Unfortunately, the treatment may thus leave contaminating agents in an active form, possibly leading to vaccine contamination by hardy infectious agents. There is thus a need for broad-spectrum inactivators which will inactivate both target microorganisms and potential contaminants. Such inactivators would also be useful for treating products such as bovine serum or trypsin, where viral inactivation is recommended [4].
Another difficulty with some inactivators is their stability. Formaldehyde is an effective inactivator, particularly at high temperatures, but it is very stable and so residues remain after inactivation. These residues can interfere with downstream testing for residual active microorganisms. Thus high dilutions are used in these tests, but this reduces their sensitivity. Moreover, the residues can interfere with immune responses in final vaccines e.g. ref. 5 suggests that formalin-inactivated vaccines might have a higher risk of causing hypersensitivity.
Finally, inactivation with formaldehyde can be reversible in some circumstances [6,7].
There is thus a need for new and improved microorganism inactivators. These should be useful against a variety of microorganisms, including hardy ones, without removing desired immunogenicity. They should also leave little or no interfering or harmful residues, and the inactivation should not be reversible. Ideally, they should also be suitable for degrading nucleic acid, and should display their activity even in the presence of aqueous media.